A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging collected vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, comprises a solenoid actuator that is under the control of a microprocessor-based engine management system.
During conditions conducive to purging, evaporative emission space that is cooperatively defined by the tank headspace and the canister is purged to the engine intake manifold through a canister purge solenoid valve connected between the canister and the engine intake manifold. The canister purge solenoid valve is opened by a signal from an engine management computer in an amount that allows intake manifold vacuum to draw volatile fuel vapors from the canister for entrainment with the combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have their evaporative emission control systems equipped with on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure, and then watching for a change in that substantially different pressure which is indicative of a leak.
It is believed fair to say that there are two basic types of diagnostic systems and methods for determining integrity of the evaporative emission space against leakage.
Commonly owned U.S. Pat. No. 5,146,902 "Positive Pressure Canister Purge System Integrity Confirmation" discloses one type: namely, a system and method for making a leakage determination by pressurizing the evaporative emission space to a certain positive pressure therein (the word "positive" meaning relative to ambient atmospheric pressure) and then watching for a drop in positive pressure indicative of a leak.
The other type makes a leakage determination by creating in the evaporative emission space a certain negative pressure (the word "negative" meaning relative to ambient atmospheric pressure so as to denote vacuum) and then watching for a loss of vacuum indicative of a leak. A known procedure employed by this latter type of system in connection with a diagnostic test comprises utilizing engine manifold vacuum to create vacuum in the evaporative emission space. Because that space may, at certain non-test times, be vented through the canister to allow vapors to be efficiently purged when the CPS valve is opened for purging of the canister, it is known to communicate the canister vent port to atmosphere through a vent valve that is open when vapors are being purged to the engine, but that closes preparatory to a diagnostic test so that a desired test vacuum can be drawn in the evaporative emission space for the test. Once a desired vacuum has been drawn, the purge valve is closed. Leakage is reflected by a loss of vacuum during the length of the test time after the purge valve has been operated closed.
In order for the engine management computer to ascertain when the desired vacuum has been drawn so that it can command the purge valve to close, and for loss of vacuum to thereafter be detected, it is known to employ an electric sensor, or transducer, that measures negative pressure, i.e. vacuum, in the evaporative emission space by supplying a measurement signal to the management computer. It is known to mount this sensor on the vehicle's fuel tank where it will be exposed to the tank headspace. For example, commonly assigned U.S. Pat. No. 5,267,470 discloses a pressure sensor mounting in conjunction with a fuel tank roll-over valve.
In one respect, the present invention is directed to a novel system and method for testing the integrity of an evaporative emission control system against leakage that is well-suited for further improving the accuracy of each of the two aforementioned basic systems and methods.
The invention provides a number of important advantages. Use of the inventive system and method in one aspect provides a time-based measurement of multiple events relating to the direct regulation of evaporative emission space test pressure, either positive or negative depending on which one of the two basic systems and methods is employed, between an upper regulating limit (URL) and a lower regulating limit (LRL). Stated another way, an aspect of the inventive system and method provides a time-based measurement of multiple events derived from an actual gas flow volume through a defined flow path substantially equal to a corresponding gas volume that has flowed through one or more leak paths in the evaporative emission space.
As a result, a system and method that embody the inventive principles is believed to be rendered more insensitive to sporadic transient events that could otherwise impair test accuracy. It is also believed to be more insensitive to other influences, such as the amount of liquid fuel in the tank when a test is being performed. By providing improved accuracy, the inventive time-based measurement of multiple events can serve to reduce the likelihood of a false indication of a leak. Moreover, the inventive system and method can serve to perform of both a "gross leak" test and a "pinched-line" test, as well as a measurement of the size of a leak.
Speaking generally of its apparatus aspect, the invention relates to an engine-powered automotive vehicle evaporative emission control system comprising an evaporative emission space for containing volatile fuel vapors, and a leak detection system for detecting leakage from the evaporative emission space, wherein leak detection system comprises: initial pressurizing means operable at the beginning of a test for creating in the evaporative emission space, an initial test pressure that differs sufficiently from atmospheric pressure to allow a leak to be detected; pressure restoring means operable after attainment of the initial test pressure in the evaporative emission space for restoring the initial test pressure in the evaporative emission space whenever the pressure changes by a predetermined amount due to a leak; and measuring means for measuring at least one of: a) a time interval required for pressure in the evaporative emission space to change from the initial test pressure by the predetermined amount; and b) a time interval required for the pressure restoring means to restore the initial test pressure in the evaporative emission space.
Speaking generally of its method aspect, the invention relates to performing a leak detection which comprises: at the beginning of a test, creating in the evaporative emission space, an initial test pressure that differs sufficiently from atmospheric pressure to allow a leak to be detected; after attainment of the initial test pressure in the evaporative emission space, restoring the initial test pressure in the evaporative emission space whenever the pressure changes from the initial test pressure by a predetermined amount due to a leak; and measuring at least one of: a) a time interval required for pressure in the evaporative emission space to change from the initial test pressure by the predetermined amount; and b) a time interval required to restore the initial test pressure in the evaporative emission space. The foregoing, along with further details, features, advantages, and benefits of the invention, will be seen in the ensuing description and claims, which are accompanied by drawings. The drawings disclose a presently preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.